Controlled-source electromagnetic geophysical surveys use man-made sources to generate electromagnetic fields to excite the earth, and deploy receiver instruments on the earth's surface, on the seafloor, in the air, or inside boreholes to measure the resulting electric and magnetic fields, i.e., the earth's response to the source excitation. FIG. 1 illustrates the basic elements of an offshore CSEM survey. A vessel tows a submerged CSEM transmitter 11 over an area of subseafloor 13. The electric and magnetic fields measured by receivers 12 are then analyzed to determine the electrical resistivity of the earth structures beneath the surface or seafloor. This technology has been applied for onshore mineral exploration, oceanic tectonic studies, and offshore petroleum and mineral resource exploration. See A. D. Chave, S. Constable, and R. N. Edwards, in Electromagnetic Methods in Applied Geophysics (ed. M. N. Nambighian), Vol. 2, 931-966, Society of Exploration Geophysicists; L. MacGregor, M. Sinha, and S. Constable, Geophy. J. Int. 146, 217-236 (2001); S. Ellingsrud, T. Eidesmo, S. Johansen, M. C. Sinha, L. M. MacGregor, and S. Constable, The Leading Edge, 972-982 (2002); T. Eidesmo, S. Ellingsrud, L. M. MacGregor, S. Constable, M. C. Sinha, S. Johansen, F. N. Kong, and H. Westerdahl, First Break 20.3, 144-152 (2002).
Electromagnetic (EM) fields are generated by the transmitter injecting currents of a chosen low-frequency periodic waveform into the earth or seawater/seafloor. For inland and airborne CSEM surveys, measurements are usually made in the frequency range of 0.1 Hz to 50 kHz. Because of the low resistivity of the seafloor, offshore CSEM surveys are usually operated in a low frequency range (0.01 Hz to 100 Hz) to achieve sufficient exploration depth. The shape of the transmitted waveform determines its frequency spectrum. In other words, the transmitter waveform controls the frequency content, distribution and relative amplitudes of the component frequencies. This follows from Fourier analysis in which any function can be expressed as a series of sine or cosine functions of different frequencies. The less the function resembles a sinusoid, the more terms, and hence the more frequencies, that are needed in the Fourier expansion to give a good approximation of the function. The lower frequencies penetrate deeper beneath the earth's surface than the higher frequencies. In current surveys, the transmitter is flown above the earth's surface by an aircraft, or towed by a vessel only once along each survey line, due to the high cost of operating the source aircraft or vessel. Because the earth contains electrical structures at all depths, the use of multiple source frequencies at uniformly high power is highly desirable so that a range of depth can be probed using only one source tow. Therefore, in order to acquire data efficiently and satisfy survey objectives as much as possible, it is desirable that the transmitter waveform have certain features. Its frequency spectrum should have multiple frequencies spread out over a frequency range (bandwidth) large enough to probe the depth range of interest. The energy transmitted at each such frequency should be as equal as possible or practical.
A variety of waveforms are available for use. The simplest one is a sinusoidal wave, but this contains only one frequency. The square wave is a simple and currently widely-used waveform, especially the symmetric square wave shown in FIG. 2A which has the same time duration for positive and negative polarities. This simple waveform has been used in CSEM work because it is easy to generate from a technological standpoint and it provides a useful range of frequencies; see L. M. MacGregor, Electromagnetic investigation of the Reykjanes Ridge near 58° North, Ph.D. Dissertation, Cambridge, 84-86 (1997). The symmetric square wave has only odd harmonics in its spectrum, and the harmonic amplitudes decrease quickly with increasing frequencies as shown in FIG. 2B. With the limitation of fixed transmitter power, it is very difficult to collect high signal-to-noise data for high harmonics unless the data are summed for a very large number of cycles. The abscissa values for FIG. 2A are normalized such that the period of the waveform is unity, and the ordinate values are normalized to have unit amplitude. The frequency values of the spectrum shown in FIG. 2B are also normalized by dividing by the fundamental frequency, i.e., the reciprocal of the repetition period of the waveform. The same type of normalization is used for all figures herein displaying waveforms and spectra.
A special waveform was designed by Constable and Cox to have the same and relatively large amplitude (power transmitted at that frequency) for the first and third harmonics (J. Geophs. Res. 101, 5519-5530 (1996)). This “Cox” waveform and its spectrum are shown in FIGS. 3A and 3B, respectively. Unfortunately the frequency band covered by these two harmonics is narrow [3:1 ratio], and amplitudes for higher harmonics decrease rapidly as the frequency increases.
Pseudo-random binary sequence (“PRBS”) waveforms are well-known, and can provide more useful frequencies that span a wider frequency band. See, for example, P. M. Duncan et al., Geophysics 45, 1276-1296 (1980); and S. L. Helwig, et al., SEG Annual Meeting Extended Abstracts, 283-285 (1999). FIG. 4A shows a PRBS generated by a shift register of degree 4, and FIG. 4B shows its spectrum. Frequencies provided by a PRBS are spaced uniformly on a linear frequency scale.
A general numerical method has been taught (Cherkaeva, E. and Tripp, A. C., SEG Annual Meeting Extended Abstracts, 438-441 (1997)) for designing an optimal time-domain transmitter waveform for a stated subsurface electromagnetic imaging problem. Obtaining the Fourier-transformed frequencies of such an optimal waveform would be readily performed by practitioners in the art. However the method of Cherkaeva and Tripp requires a priori specification of the target properties, and depth, and in the example taught for probing a single buried layer, the waveform contains a single frequency whose amplitude varies sinusoidaly in time.
In a related but not directly applicable technology relative to the invention described herein, Hombostel and Thompson (U.S. Pat. No. 6,477,113) teach the use of a specialized electromagnetic source waveform for electroseismic geophysical applications, in which the spectrum of the source waveform is designed so as to have minimum correlation with frequencies outside the broadcast source spectrum.